Thanks to globalization, crimes seem to be mimicked around the planet, but I haven´t heard of this particular type of mugging activity. What you describe, though, is holding up a bank customer who´s just withdrawn money.

I don't think that there's a specific term to differentiate between "mugging at an ATM" and mugging in any other location.

If Mel07 is accurate in her (his?) very succinct definition, then another verb that applies to this situation is ambush (emboscar)... but this would be a more liberal usage. I think mug is the best option.